The Aurora or some equivalent probably exists since the US spends an awful lot of mone of scramjet development.
Scramjets are ramjet engines where the air passes through the engine without being slowed down to subsonic speeds, therefore it doesn't waste it's energy and gives of more thrust.
Ramjets are jet engines that have no moving parts - they only start working at roughly 2 machs. Instead a turbine it is the flow of air itself into the engine that compresses it.
The Aurora will probably use some scramjet engine, however since these only work on high speeds it will probably need a different type of engine for taking of a reaching cruising height - Pulse Detonation Engines are good candidate since their output is still good in the area where scramjets start to work.
